Cellulosic fibrous structures, such as paper towels, facial tissues, and toilet tissues, are a staple of every day life. The large demand and constant usage for such consumer products has created a demand for improved versions of these products and, likewise, improvement in the methods of their manufacture. Such cellulosic fibrous structures are manufactured by depositing an aqueous slurry from a headbox onto a Fourdrinier wire or a twin wire paper machine. Either such forming wire is an endless belt through which initial dewatering occurs and fiber rearrangement takes place. Frequently, fiber loss occurs due to fibers flowing through the forming wire along with the liquid carrier from the headbox.
After the initial formation of the web, which later becomes the cellulosic fibrous structure the papermaking machine transports the web to the dry end of the machine. In the dry end of a conventional machine, a press felt compacts the web into a single region cellulosic fibrous structure prior to final drying. The final drying is usually accomplished by a heated drum, such as a Yankee drying drum.
One of the significant aforementioned improvements to the manufacturing process, which yields a significant improvement in the resulting consumer products, is the use of through-air drying to replace conventional press felt dewatering. In through-air drying, like press felt drying, the web begins on a forming wire which receives an aqueous slurry of less than one percent consistency (the weight percentage of fibers in the aqueous slurry) from a headbox. Initial dewatering takes place on the forming wire, but the forming wire is not usually exposed to web consistencies of greater than 30 percent. From the forming wire, the web is transferred to an air pervious through air drying belt.
Air passes through the web and the through-air-drying belt to continue the dewatering process. The air passing the through-air-drying belt and the web is driven by vacuum transfer slots, other vacuum boxes or shoes, predryer rolls, etc. This air molds the web to the topography of the through-air-drying belt and increases the consistency of the web. Such molding creates a more three dimensional web, but also creates pinholes if the fibers are deflected so far in the third dimension that a breach in fiber continuity occurs.
The web is then transported to the final drying stage where the web is also imprinted. At the final drying stage, the through air drying belt transfers the web to a heated drum, such as a Yankee drying drum for final drying. During this transfer, portions of the web are densifted during imprinting to yield a multi-region structure. Many such multi-region structures have been widely accepted as preferred consumer products. An example of an early through-air-drying belt which achieved great commercial success is described in U.S. Pat. No. 3,301,746, issued Jan. 31, 1967 to Sanford et al.
Over time, further improvements became necessary. A significant improvement in through-air-drying belts is the use of a resinous framework on a reinforcing structure. This arrangement allows drying belts to impart continuous patterns, or, patterns in any desired form, rather than only the discrete patterns achievable by the woven belts of the prior art. Examples of such belts and the cellulosic fibrous structures made thereby can be found in U.S. Pat. Nos. 4,514,345, issued Apr. 30, 1985 to Johnson et al.; 4,528,239, issued Jul. 9, 1985 to Trokhan; 4,529,480, issued Jul. 16, 1985 to Trokhan; and 4,637,859, issued Jan. 20, 1987 to Trokhan. The foregoing four patents are incorporated herein by reference for the purpose of showing preferred constructions of patterned resinous framework and reinforcing type through-air-drying belts, and the products made thereon. Such belts have been used to produce extremely commercially successful products such as Bounty paper towels and Charmin Ultra toilet tissue, both produced and sold by the instant assignee.
As noted above, such through-air-drying belts used a reinforcing element to stabilize the resin. The reinforcing element also controlled the deflection of the papermaking fibers resulting from vacuum applied to the backside of the belt and airflow through the belt. The early belts of this type used a fine mesh reinforcing element, typically having approximately fifty machine direction and fifty cross-machine direction yarns per inch. While such a fine mesh was acceptable from the standpoint of controlling fiber deflection into the belt, it was unable to stand the environment of a typical papermaking machine. For example, such a belt was so flexible that destructive folds and creases often occurred. The fine yarns did not provide adequate seam strength and would often burn at the high temperatures encountered in papermaking.
Yet other drawbacks were noted in the early embodiments of this type of through-air-drying belt. For example, the continuous pattern used to produce the consumer preferred product did not allow leakage through the backside of the belt. In fact, such leakage was minimized by the necessity to securely lock the resinous pattern onto the reinforcing structure. Unfortunately, when the lock-on of the resin to the reinforcing structure was maximized, the short rise time over which the differential pressure was applied to an individual region of fibers during the application of vacuum often pulled the fibers through the reinforcing element, resulting in process hygiene problems and product acceptance problems, such as pinholes.
A new generation of patterned resinous framework and reinforcing structure through-air-drying belts addressed some of these issues. This generation utilized a dual layer reinforcing structure having vertically stacked machine direction yarns. A single cross-machine direction yarn system tied the two machine direction yarns together.
For paper toweling, a relatively coarse mesh, such as thirty-five machine direction yarns and thirty cross-machine direction yarns per inch, dual layer design significantly improved the seam strength and creasing problems. The dual layer design also allowed some backside leakage to occur. Such allowance was caused by using less precure energy in joining the resin to the reinforcing structure, resulting in a compromise between the desired backside leakage and the ability to lock the resin onto the reinforcing structure.
Later designs used an opaque backside filament in the dual layer design, allowing for higher precure energy and better lock-on of the resin to the reinforcing structure, while maintaining adequate backside leakage. This design effectively decoupled the tradeoff between adequate resin lock-on and adequate backside leakage in the prior art. Examples of such improvements in this type of belt are illustrated by U.S. patent application Ser. No. 07/872,470 filed Jun. 15, 1992 in the names of Trokhan et al. now U.S. Pat. No. 5,334,289. Yet other ways to obtain a backside texture are illustrated by U.S. Pat. Nos. 5,098,522, issued Mar. 24, 1992 to Smurkoski et al.; 5,260,171, issued Nov. 9, 1993 to Smurkoski et al.; and 5,275,700, issued Jan. 4, 1994 to Trokhan, which patents and application are incorporated herein by reference for the purpose of showing how to obtain a backside texture on a patterned resin and reinforcing structure through-air-drying belt.
As such resinous framework and reinforcing structure belts were used to make tissue products, such as the commercially successful Charmin Ultra noted above, new issues arose. For example, one problem in tissue making is the formation of small pinholes in the deflected areas of the web. It has recently been learned that pinholes are strongly related to the weave configuration of the reinforcing element of the patterned resinous through-air-drying belt.
Standard patterned resinous through-air-drying belts maximize the projected open area, so that airflow therethrough is not reduced or unduly blocked. Patterned resinous through-air-drying belts common in the prior art use a dual layer design reinforcing element having vertically stacked warps. Generally, the wisdom has been to use relatively large diameter yarns, to increase belt life. Belt life is important not only because of the cost of the belts, but more importantly due to the expensive downtime incurred when a worn belt must be removed and a new belt installed. Unfortunately, larger diameter yarns require larger holes therebetween in order to accommodate the weave. The larger holes permit short fibers, such as Eucalyptus, to be pulled through the belt and thereby create pinholes. Unfortunately, short fibers, such as Eucalyptus, are heavily consumer preferred due to the softness they create in the resulting cellulosic fibrous structure.
This problem can be overcome by adding more yarns per inch woven in the same pattern. However, this "solution" reduces the open area available for air flow. If the yarns are made smaller to reopen the open area, the flexural rigidity and integrity of the reinforcing structure of the belt is compromised and the belt life is thereby reduced. Accordingly, the prior art required a trade-off between the necessary open area (for airflow) and fiber diameter (for pinholing and belt life).
One attempt to achieve both good fiber support, and the flexural rigidity and belt integrity necessary to achieve a viable belt life was to use a combination of large and small machine direction yarns. The large diameter yarns are disposed on the reinforcing layer for fabric durability, and the smaller diameter machine direction yarns are stacked on the web facing layer for fiber support and pinhole reduction. Furthermore, a small machine direction yarn in the first layer may be placed between large machine direction yarns of the second layer for added fiber support. This attempt still did not produce wholly satisfactory results in pinhole reduction efforts due to a lack of planarity. Accordingly, it is necessary to turn to yet a different parameter than those utilized above to decouple the trade-offs required by the prior art.
One attempt to find a different parameter was to add a machine direction yarn between each pair of stacked machine direction yarns, so that a single cross-machine direction yarn tied together stacked machine direction yarns. However, one problem this attempt encountered was the machine direction yarns not supported immediately thereunder by another yarn tended to sag--increasing pinholing. Additionally, the cross-machine direction yarns which tied the two layers together went from the extreme of one layer to the extreme of the other layer. This deviation from planarity also increased pinholing.
A second attempt increased the tie frequency of the cross-machine direction yarns from a six shed to a four shed. However, similar problems occurred--including sagging of the machine direction yarns of the upper layer which were stacked with the machine direction yarns of the lower layer, due to either inadequate support from the other yarns, or due to being pulled towards the second layer by the cross-machine direction yarns.
These approaches were not successful. Clearly yet another approach was necessary.
Likewise, the weave pattern must be applicable to press felts. Press felts dewater a cellulosic web by compaction. Suitable press felts may be made in accordance with U.S. Pat. Nos. 3,652,389 issued Mar. 28, 1972 to Helland; 4,752,519 issued Jun. 21, 1988 to Boyer et al.; and 4,922,627 issued May 8, 1990 to Romero Hernandez, which patents are incorporated herein by reference for the purpose of showing how to make a press felt according to the present invention.
The necessary approach recognizes that pinholing in a through-air-drying belt and fiber loss in a forming wire are unexpectedly related to the yarns that support the fibers--rather than the open spaces between the yarns. The web facing yarns must remain close to the top plane of the first layer, to provide adequate fiber support. Still, the weave pattern must accommodate large diameter yarns in order to provide adequate belt life.
Accordingly, it is an object of this invention to provide a forming wire which reduces fiber loss and non-uniform fiber distribution in specific areas of the resulting product. It is another object of this invention to provide a patterned resinous through-air-drying papermaking belt which overcomes the prior art trade-off of belt life and reduced pinholing. Additionally, it is an object of this invention to provide an improved patterned resinous through-air-drying belt having sufficient open area to efficiently use during manufacturing. It is also an object of this invention to provide a patterned resinous through-air-drying belt which produces an aesthetically acceptable consumer product comprising a cellulosic fibrous structure.